India is the world’s No. 1 milk producer. Unfortunately, rural India is not well served by the electrical grid -- creating problems for milk producers who need to keep their product cool.

In much of India, milk is brought by farmers to small, village-based collection centers. If the milk spoils before being collected from the center by the dairy, the farmer won’t get paid. What’s more, if a farmer misses that day’s pickup from the dairy, the product is unlikely to remain fresh without reliable refrigeration.

The milk industry needs reliable refrigeration, but that’s not easy with a highly unpredictable electrical grid.

The collection centers have a choice: risk spoiled product or invest in a diesel generator for when the grid fails. The second option is not only bad news for the environment -- it also considerably inflates the operating cost of refrigeration.

Promethean Power Systems has come up with a solution to the problem. The company is manufacturing milk-chilling units connected to a patent-pending thermal battery that uses a phase-change material to store 28 kilowatt-hours of energy in the form of ice.

When electricity is available, ice forms in the thermal battery. When the grid supply is interrupted, the melting ice maintains the temperature of the chilling unit to ensure the milk -- up to 2,110 pints of it in each chiller -- rapidly drops in temperature. More importantly, it remains cool, and far less likely to spoil, before being picked up by trucks from the dairy company.

According to Sorin Grama, a graduate of the Massachusetts Institute of Technology and Promethean’s CTO and co-founder, each unit supports the production from 20 to 30 farmers in a typical Indian village. Each unit, he says, costs around $8,000 to $10,000, depending on the model.

“Our systems are just a bit more expensive to buy than a regular diesel generator system, but the savings in operating costs make up the difference in less than one year. Our operating costs can be half of the operating cost of an equivalent chiller running with partial diesel power,” claimed Grama.

Around 150 units have been sold so far, giving the company an installed base of around 2.5 megawatt-hours of distributed thermal energy storage.

The dependability of the units and their ability to rapidly chill milk to an optimal storage temperature mean that collection centers are far less likely to face problems with spoiling, giving them and the dairy companies a more reliable business model.

“Farmers get paid well, get paid on time, and, in some cases, they get assistance to purchase more cattle to produce more milk. Thus, their quality of life increases,” said Grama.

Although Promethean Power Systems originated in the Boston area in 2007 -- and still maintains an office in nearby Somerville, Mass. -- the vast majority of its activity, including manufacturing, now takes place in India.

Grama and his colleagues went to India on a mission to improve the lives of people. But the journey was not without setbacks.

“We experimented with many technical approaches to the problem and failed a few times before stumbling on this thermal battery solution. Our initial orders came in 2011. Our first big commercial order came in 2013," he said.

The key to the success of the technology was to create a thermal battery that behaved like an electrochemical cell, with a predictable, steady discharge of energy to ensure a stable temperature.

And just like a regular battery, multiple Promethean thermal units can be placed in parallel to multiply the system’s capacity to store energy and provide for longer periods of refrigeration. Unlike a conventional cell, the thermal battery does not rely on electrochemical reactions, making it highly stable. The oldest system has been in operation for three years, so far, and Grama speculates that the thermal battery component will have a seven-year lifespan on average.

Now that Promethean has proven the value and commercial viability of its technology, the company is looking to increase sales, production and the efficiency of the manufacturing process -- which will help make its units even more affordable.

Are there possibilities for offering a truly off-grid solution, powered by renewables?

It turns out that the company has already tried that approach, but it makes little economic sense in India, as very few villages are totally without electrical power for at least some of the day.

But Grama does see solar as a potential way to reduce power costs, and the company plans to offer a solar-hybrid option to customers.

“We are just starting to experiment with an off-grid solar system to power the milk collection center. This will be a small 1- to 1.5-kilowatt PV system. The thermal battery itself will not be charged by solar because it requires too much power, but all other loads, such as the coolant pump, can be powered by solar and a small electrical battery bank,” said Grama. Currently, a lead-acid car battery provides the energy storage for these additional loads.

Using solar to go off-grid entirely could be problematic in the case of chilling milk -- all it takes is a few cloudy days and the system would cease to be effective. However, situations in which maintaining a narrow temperature range is less critical, such as fruit and vegetable storage, may be future markets.